No, no, no.
gnome and KDE would be provided by different packages (actually, lots of packages for gnome and for KDE).
If you want to use a different Desktop Environment, you can install that package.
But my distro uses gnome by default.
This is just a default
Now if i run the apt full-upgrade command, the default DE that comes with it is Gnome and not KDE.
No. It will upgrade all packages. From gnome, from KDE or from the hundreds of developers that make software not associated with any of those projects.
Wouldn't that also put back Gnome and i would have to uninstall Gnome manually again, because the repository contains Gnome?
No. In fact, you wouldn't need to uninstall gnome to use KDE, both can coexist (you may uninstall gnome if you want to save disk space, but it's not needed)
gnome might get reinstalled if the upgrade added gnome as a dependency. You would see it on the list of packages to upgrade, though.
1.) How do i stop Gnome from installing when i do a apt full-upgrade? (Since i don't want Gnome)
Why do you think it will be installed?
2.) How do i go about managing my KDE package (i.e updating it).
Do i also do a apt-mark hold on the KDE package just to "prevent any potential tamper" whenever i do apt full-upgrade? and then just apt-get update && apt-get upgrade KDE commands to update it?
apt full-upgrade will upgrade the KDE packages with no issue. Setting holds on KDE packages could prevent the upgrade to actually work correctly on certain cases, for no benefit.